Predaceous Diving Beetles

Animals, July, 2000 by Sy Montgomery

Just when you thought it was safe to go back into the water ...

At the edge of the pond, where the mud is soft and warm, where tadpoles and newts dart among the flowering pickerelweed and frogs grin up from the shallows, an innocent-looking brown beetle, its Volkswagen shape neatly adapted for aquatic life, oars gently through the water with bladelike legs.

Don't be deceived, cautioned Mount Holyoke College zoologist Ann Haven Morgan. "Although no insect looks more gentle and satisfied," she observed in her 1930 Field Book of Pond and Streams, "none is more fierce and voracious."

In ponds, in pools, in the sidewaters of streams, legions of these beetles seize and gobble up almost anything that moves. Not just mosquito larvae (although they eat these by the dozen). Not just dragonfly larvae (ferocious predators themselves). These things eat salamanders. They eat fish. They eat tadpoles. Not even adult frogs are safe from the predaceous diving beetle.

But wait, aren't critters with backbones--such as fish and frogs--supposed to eat bugs, and not the other way around?

"This isn't the way we think the world is supposed to be," observes Robert Roughley, an entomologist at the University of Manitoba. The predaceous diving beetle seems to defy the rules of nature. It "breathes" through its back end, carries its air supply like an Aqua-Lung beneath the water, and can fly as well as dive.

Good thing they're not any bigger. Fortunately, they nip you only if you bother them--or one gets stuck in your bathing suit. There are some 500 different kinds of predaceous diving beetles in North America, and the largest grow to an inch and a half--a relative giant among insects (the average insect is only 3/25ths of an inch long).

Only one predator in the pond is more fearsome than an adult predaceous diving beetle: a baby predaceous diving beetle. The beetle's larval form, called a water tiger, can grow three inches long. It looks somewhat like a shrimp, with a head borrowed from somebody's nightmare. The sickle-shaped, hollow jaws clutch their prey and funnel flesh-digesting drool into the victim until the water tiger literally sucks it dry. It can eat 20 tadpoles a day.

"You can see various science fiction stories were inspired by these," says Donald Chandler, professor and curator of zoology at the University of New Hampshire. "You just know the scriptwriter has seen them." The life of this creature is weirder yet--a real-life sci-fi episode that has lasted 180 million years, now playing at a pond or stream near you.

Northern New England is one of the predaceous diving beetle capitals of the world. More individuals of more different species of this beetle are found along the 49th parallel, where the United States and Canada meet, than anywhere else on earth, even the tropics. Here you may find 10 to 15 different species in an area of pond the size of a tabletop. It's hard to tell the species apart, though. Beetles are usually identified by observing differences in the male's genitalia, which sport all sorts of uncomfortable-looking protuberances. Unfortunately--at least for entomologists trying to classify them--male predaceous diving beetles lack these embellishments.

At least one species appears to have disappeared: the elusive diving beetle, which was last seen by Phil Darlington, a Harvard Museum of Comparative Zoology entomologist and biogeographer, at his Rumney, New Hampshire, summer house on April 22, 1926. The elusive beetle remains true to its name, despite Chandler's attempt to rediscover it one recent spring, as part of the New Hampshire Globally-Historic Insects Survey.

Any species of predaceous diving beetle can provide the "sweet sensation of horror," that "shivery fascination with monsters" that so delight the likes of Harvard's E. O. Wilson and his entomological colleagues. To see for yourself, inspect the shallows at a pond or pool where the water is clear but also sports some vegetation. This is where the beetles like to hunt. Watch the top two inches of the water for movement. In a few minutes you may see a black or brownish black beetle, sometimes outlined with dull yellow, floating backwards to the surface. Then it will lift its wing covers, collect a silver bubble of air, and dive again. This is how the beetle replenishes its air supply.

The beetle can stay underwater many minutes because it carries this air bubble like a diver's air tank. Some species of diving beetle carry the bubble at the tip of the abdomen, where the trapped air acts like the gill of fish. As the beetle consumes the oxygen, dissolved oxygen from the water diffuses into the bubble to replace it. In this way the insect extends its air supply. And this is fortunate, for the beetle does its best work beneath the water--hunting, eating, mating, and even grooming itself there.

The beetle's toilette brings to mind a sunbather applying tanning oil on the beach--except that it's all done underwater. To keep free of bacteria and fungi, this insect exudes two drops of bluish white fluid from the area near its wing covers, and then rubs it all over itself with its legs and antennae. It turns out this stuff is a steroid compound that also chemically helps the insect break through the surface tension of the water and repels would-be predators. Roughley discovered this last feature when he was moving beetles around with an aspirator and mistakenly sucked one into his mouth. He reacted like a fish would and spat it out--but too late. His mouth was already numb from the chemical.